Millions of doses of an unlicensed flu jab have been bought for use in the UK in a bid to keep up with demand for vaccines.
Britain has authorised the temporary use of Flublok – which is licensed in the US – with health chiefs insisting it is safe and effective.
Ministers have promised the largest vaccination programme in history, with jabs to be offered to healthy people aged between 50 and 64 as well as to older people, children and those with underlying health conditions.
However, the Royal College of GPs has raised concerns about shortages of jabs, with some practices saying they have been left waiting up to a month for supplies. A number of high street pharmacies ran out of doses in September amid unprecedented demand.
More than two million doses of Flublok have been purchased by the UK Government under emergency measures.
Professor Jonathan Van-Tam, the deputy chief medical officer, said: "As we approach the winter and cases of Covid-19 continue to rise, it is crucial we double-down on efforts to vaccinate as many people as possible from flu. We have increased the number of people eligible for free flu jabs this year to reduce all avoidable risks and protect people from illness.
"Flublok has been in regular use in the US, and the evidence shows that it is an excellent product. I want to reassure everyone that all vaccines have undergone robust clinical trials and rigorous checks by the regulator to ensure they are safe, effective and of a high quality."
Flublok received authorisation for temporary supply from the Medicines and Healthcare products Regulatory Agency (MHRA). The MHRA has the power to take such a step when it is satisfied that a medicine is safe and effective in response to a public health threat.
The regulator has been given extra powers in the course of the pandemic, such as being able also to temporarily authorise any coronavirus vaccine that meets safety and quality standards before it has received a full licence. The Flublok vaccine has been used in the US for the past three winters.
Health officials said a "physically and biologically similar vaccine" called Supemtek was recommended for approval by the European Medicines Agency in September.
All vaccines, including Flublok, undergo three stages of clinical trials and are assessed by the regulator for safety, effectiveness and quality before they are given to patients, they stressed.
A free flu vaccine is available to people aged 65 and older, pregnant women and those with some pre-existing conditions, all school year groups up to Year Seven and the household contacts of those on the NHS shielded patient list.
Officials said they would open the programme to those aged 50 to 64 without underlying conditions once vaccination of the most at-risk groups is well underway.
Dr Christian Schneider, the interim chief scientific officer at the Medicines and Healthcare products Regulatory Agency, said: "Protecting health and saving lives is at the heart of all our work. The MHRA assessed Flublok against safety, quality and effectiveness standards and sought advice from the Government's independent expert scientific advisory body, the Commission on Human Medicines.
"We are satisfied that this vaccine protects against flu and meets high standards of safety and quality."